So very stuck...

Post #12 about using MVA is veeeeeeeeeeeeeery good advice. Maybe cut back the initial to 10* & add 14* [ at idle ] with MVA. An adjustable VA unit is also reqd, screw Allen Key fully CW.

If you are not using VA [ either PVA or MVA ], then that is also contributing to the problem because the extra cruise timing of VA keeps the engine cooler. Detonation loves heat.....
I will bet if you tested the engine when cold, the noise is not there or reduced.

More on MVA benefits, scroll down to post #6.

www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html